Positive acting photoresist compositions which give rise to positive resist images are well known in the art. These compositions generally comprise an alkali-soluble polymer such as a novolac resin and a photoactive component (PAC) containing diazo and keto groups on adjacent positions in a benzenoid structure, such as for example, 2-diazo-1,2-naphthoquinone-4- or 5-sulfonic acid, and possibly a polyhydroxy compound, such as a polyhydroxybenzophenone. Exposure of these photoresist compositions imagewise to actinic light serves to convert the diazo-keto configuration of the PAC to a carboxylic acid-containing moiety. Hence, the portions of photoresist which have been exposed to the actinic radiation are rendered soluble in alkali. The desired photoresist image is then developed using alkaline developers.
The demand for microelectronic and integrated circuits with higher densities is requiring the use of optical lithography for 0.5 to 0.6 micron geometries and requiring a dramatic improvement in photoresist resolution and also requiring more exacting control over the sidewall profiles of the images. Various different approaches could be employed to achieve positive (less than 90.degree.), vertical (90.degree.) or negative (above 90.degree.) sidewall profiles. When printing images employing the aforementioned small geometries, the sidewall angle of the resist image plays an important role. The critical dimension (CD) change between the top and bottom of a 1.2 micron thick resist is 0.042 microns for each degree change of sidewall angle. At 0.5 micron geometries, this is a dramatic 8.4% change. A key to high yield production of such devices at these small geometries is therefore very tight control over sidewall profiles.
Positive photoresists typically form sidewall angles upon development that are less than 90.degree.. This generally occurs because the exposing light has more intensity at the resist surface and less at the bottom substrate surface. Also, the length of contact of the resist to the developer is more at the top than the bottom of the resist layer. Exposure of positive resists to light causes acid formation and bleaching of the PAC, and because of the residual absorption of the remaining PAC through the thickness of the resist a gradient of acid is formed in which the top surface has a higher acid concentration than the bottom. This is in turn, further modified by standing waves. As development of exposed resist proceeds, the top layer is immediately in contact with the developer while the bottom contacts the developer only when the developer reaches that thickness, a typical time difference of 30 to 60 seconds. These two factors contribute to positive sidewall angles of less than 90.degree.. These two factors can only be minimized, since they exist in every positive photoresist, but not eliminated.
The only way to counteract the formation of a positive sidewall angle is through increased surface inhibition of the resist or the formation of an increasing dissolution gradient through the resist thickness. This can be done by developer treatment before exposure, by UV flood exposure while the resist is heated wherein crosslinking occurs in the top layer of resist, or by special design of the resist components that have this feature built into the resist formulation. The most desirable sidewall profile in microlithography during normal processes is vertical or as close to vertical as possible. This sidewall angle allows easy inspection and measurement, and helps in image transfer to the substrate during plasma etch. Keeping the sidewall angle as close to vertical as possible across various exposure energies and various focus settings is a challenge of every resist manufacturer and the desire of every advanced user.
Moreover, a further problem is encountered due to the presence of reflected light during the imagewise exposure of the photoresists. Reflections off layers, such as metallized layers or resist surface irregularities, results in fine lines disappearing. Conventional photoresists are very susceptible to such surface reflections which destroy or degrade the pattern created in the photoresist during imagewise exposure of the photoresist. Another problem caused by the aforesaid is reflective notching or undercutting of the sidewalls due to light scattering from reflective topography.
It is therefore an object of this invention to provide photoresist compositions which eliminate or substantially eliminate undercutting or reflective notching due to light scattering from photoresist topography. A further object of this invention is to provide photoresist compositions which control sidewall profiles to essentially vertical, increase exposure latitude and reduce or substantially eliminate undercutting or reflective notching, especially for photoresists employed to produce devices of small geometries of about 0.5-0.6 microns.